The question of the Decapod's colour has come up in the GER Society from time to time. It is generally accepted now that it never appeared in blue, although various "artists' impressions" have shown it thus.
Not the only example of this I have seen, and not only with railways. I have a coloured 'publicity' postcard showing a BAC Concorde in BOAC blue and white, whereas they were so late that BOAC had become BA by the time they entered service!
However, back to the original question. GER gold lettering and numbering transfers are available in both 4mm and 7mm scales from:
Guilplates, 32 Wodeland Avenue, Guildford GU2 4JZ. e-mail firstname.lastname@example.org
Hope this helps